Durable corrosion and ultraviolet-resistant silver mirror

ABSTRACT

A corrosion and ultra violet-resistant silver mirror for use in solar reflectors; the silver layer having a film-forming protective polymer bonded thereto, and a protective shield overlay comprising a transparent multipolymer film that incorporates a UV absorber. The corrosion and ultraviolet resistant silver mirror retains spectral hemispherical reflectance and high optical clarity throughout the UV and visible spectrum when used in solar reflectors.

This application claims the benefit of provisional application Ser. No.60/095,884 filed Aug. 10, 1998.

TECHNICAL FIELD

This invention relates to specular silver mirrors. More particularly, itrelates to a durable corrosion and ultraviolet-resistant silver mirrorfor use in solar reflectors.

BACKGROUND ART

Insufficient weather protection and ultraviolet degradation areproblems, which are encountered when using solar reflectors made of aflexible specular silver mirror. When used outdoors, these mirrors mustbe durable and ultraviolet light (UV) resistant in order to retain theirdimensional stability, aesthetic appearance, and specular-reflectance inthe visible, ultraviolet, and near infra-red wavelengths.

Specular-reflectance is provided in a flexible silver mirror through asilvered composite lamina, having a thin layer of silver vapor-depositedon the surface of a flexible polyester substrate. Silver is thepreferred metal because its reflectivity is substantially higher thanthat of other metals, such as aluminum. To retain specular reflectanceover time, the prior art has focused on the application of advancedadhesives and protective films, layered over the polyester substrate andsilver layer, to protect the mirrors from abrasion, weathering, andultraviolet degradation.

Early techniques used to protect solar mirrors from abrasion,weathering, and ultraviolet degradation were developed with aluminummirrors. For example, in U.S. Pat. No. 4,307,150, a solar reflector isdisclosed wherein an opaque aluminum surface, vapor-deposited on aflexible polyester support-sheet, is protected from corrosion andweathering with an inter-polymer layer of acrylate or methacrylatecopolymers. The support sheet consists of a biaxially orientedpolyethylene terephthalate lamina having conventional slip agents, tofacilitate winding, and a second polyethylene terephthalate lamina whichcontains no slip agent.

Silver is higher in specular reflection than aluminum. Thus, the logicalassumption had been to substitute silver for aluminum in the solarreflector described above. However, this approach has been reported, inU.S. Pat. No. 4,645,714, to result in two undesirable phenomena. First,silver is susceptible to corrosion either through the development of pinholes in the acrylate coating or along peripheral portions of thesilver-coated film. Second, a thin layer of silver, unlike a thin layerof aluminum, has a spectral window through which ultraviolet (“UV”)light readily passes. The peak transmission of this light is at 320 nm,and sunlight contains ultraviolet light at this wavelength. Thetransmission of ultraviolet light through the silver layer degrades theunderlying polyester substrate causing bubbles in the adhesives,commonly used to adhere the substrate to a rigid support. Thisdegradation and bubbling reduces the aesthetic and specular functionalproperties of the solar mirror.

Corrosion inhibitors and UV absorbers, incorporated into the adhesivesor protective film coatings overlaying a polyester and silver mirrorsubstrate, have been used to retain these functional properties.However, while corrosion inhibitors do reduce corrosion, they frequentlyimpart an unacceptable color to the mirror, over time, and do not blockthe ultraviolet light. In contrast, when ultraviolet light absorbers areincorporated into a protective polymer overlay, the rate of polyestersupport degradation is lessened, but silver corrosion is aggravated.Thus, attempts have been made to isolate the corrosion inhibitor andultraviolet absorber elements from the mirror's reactive components inorder to eliminate these undesirable effects.

In Roche, U.S. Pat. No. 4,645,714, a corrosion resistant silver mirroris disclosed wherein a corrosion inhibitor, and an ultraviolet absorberare each incorporated into separate thin overlays of an acrylateinter-polymer paint. The specular reflective mirrors are formed byvapor-depositing silver over a polyester support film. Ultravioletdegradation of the polyester support, and consequent bubbling of theunderlying adhesive, is reduced by incorporating UV absorbers in asecond polymer coating that is applied over a first polymer coating,which incorporates a corrosion inhibitor. The first polymer coating isapplied directly over the silver reflective surface. The polymericsubstrate, a coextruded biaxially oriented polyester foil, comprises:(1) a polyethylene terephthalate lamina containing conventional slipagents to facilitate winding; and (2) a polyethylene terephthalatelamina containing no slip agent, which results in an optically-smoothexposed surface. The silver specular reflective layer overlies thesmooth surface, of the coextruded film, and is bonded thereto. Layeredover the silver is a first acrylate or methacrylate inter-polymercoating, having a 0.5 to 2.5% glycol dimercaptoacetate dispersant, whichserves as a coupling agent, primer, and corrosion inhibitor. Thiscoating weighs 1–4 g/m². Overlying the first acrylate coating, is asecond acrylate coating containing an ultraviolet absorber effectivethroughout the 300–400 nanometer range. The weight of the second coatingis 4–8 g/m². Inclusion of the corrosion inhibitor and the UV absorberinto separate layers is designed to keep the UV absorber out-of-contactwith the silver, and to avoid any corroding effect. On the opposite sideof the coextruded polyester support is a uniform coating, weighing about10–15 g/m², of a tacky and pressure-sensitive adhesive (95:5 isooctylacrylate:acrylamide copolymer). A conventional release liner, such as asilicone-coated polyester film, may be used to protect the adhesiveprior to use. The disclosure of U.S. Pat. No. 4,645,714 is incorporatedby reference as though fully set forth herein.

It is believed, however, by Hutchinson, U.S. Pat. No. 5,118,540, thatthe reflective films described in U.S. Pat. Nos. 4,307,150, and4,645,714 are generally unsuitable for solar energy applications. Underoutdoor conditions, the thin acrylate flood coat of these films tends toweather poorly and to quickly erode. These coatings thus offer aninsufficient protective barrier to abrasion and moisture. Where thesolar mirror comprises a substrate having a polyester support sheet anda layer of silver, as the outer acrylate flood coat, containing UVabsorbers, erodes, ultraviolet light degrades the polyester support, andthe mirror's aesthetic appearance and optical efficiency fail. In orderto mitigate this problem, Hutchinson discloses the use of corrosioninhibitors and ultraviolet absorbers in an adhesive, which is used tobond an abrasion and moisture resistant fluorocarbon protective coatingover a polyester and silver mirror substrate. The relevant embodiment,therein, describes a corrosion and ultraviolet light resistant flexiblereflective film, where the respective inhibitors and absorbers areincorporated into separate coatings of an adhesive. A thin layer ofsilver is vapor-deposited on a flexible polyester support sheet,producing the specular silver surface. The adhesive is used to bond afluorocarbon film, having an abrasion and weather resistant function, tothe surface of the silver. The adhesive is applied in two separatelayers. The first adhesive layer is adjacent to the silver deposit andcontains a corrosion inhibitor. The second adhesive layer contains a UVabsorber and overlays the first adhesive layer for use in bonding thefluorocarbon protective film to the silver surface. The use of adhesivesto bond the fluorocarbon film to the silver surface is a requiredelement of this construction, because fluorocarbon films do not bond tometal surfaces. However, this construction is not without itsdeficiencies when used, over time, as a solar mirror. Under ultravioletlight, the application of advanced adhesives (those incorporating UVabsorbers and corrosion inhibitors) directly onto a silver substrate hasresulted in degradation of the silver/adhesive interface. When silver isadhered directly to acrylic films tunnel and delamination failures havealso occurred. Moreover, fluorocarbon protective films, without UVabsorbers, often provide an insufficient weather resistant shield. Forthese reasons, it is believed that these silver mirrors when applied assolar reflectors remain lacking in long-term durability, which resultsin a loss of optical efficiency and aesthetic appearance.

In view of the foregoing considerations, a need therefore exists for adurable silver mirror, which is corrosion and weather resistant,effectively screens ultraviolet light, and retains its specular opticalefficiency and aesthetic appearance when used as a component in solarreflectors.

SUMMARY

It is therefore an object of the invention to provide a durable silvermirror for use in solar reflectors.

It is another object of the invention to provide a durable silvermirror, which is corrosion and weather resistant, effectively screensultraviolet light, and retains its specular optical efficiency andaesthetic appearance when used as a component in solar reflectors.

It is yet another object of the invention to provide a method for makinga flexible silver mirror for use in solar reflectors.

The foregoing specific objects and advantages of the invention areillustrative of those which can be achieved by the present invention andare not intended to be exhaustive or limiting of the possible advantageswhich can be realized. Thus, those and other objects and advantages ofthe invention will be apparent from the description herein or can belearned from practicing the invention, both as embodied herein or asmodified in view of any variations which may be apparent to thoseskilled in the art.

Briefly, the invention provides, in a silver mirror having a polymericsubstrate, a thin specular-reflective silver layer overlying thesubstrate and bonded thereto, and a thin protective layer offilm-forming polymer overlying the exposed surface of the silver layer,the protective layer firmly adherently bonded thereto, the improvement,comprising: an ultraviolet absorbing polymer film adhered to the exposedsurface of the protective layer.

Unless specifically defined otherwise, all technical or scientific termsused herein have the same meaning as commonly understood by one ofordinary skill in the art to which this invention belongs. Although anymethods and materials similar or equivalent to those described hereincan be used in the practice or testing of the present invention, thepreferred methods and materials are now described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the simplest form of the invention.

FIG. 2 is a cross-sectional view of a preferred form of the invention.

FIG. 3 is a cross-sectional view of another preferred form of theinvention.

FIG. 4 is a plot of the measured Spectral Hemispherical Reflectance, asa function of wavelength, which illustrates the optical clarity of theembodiment according to FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a durable silver mirror, and a method formaking the mirror, having weather and corrosion resistance and effectiveultraviolet screening. These features provide long-term durability and aretention in specular optical-efficiency and aesthetic. These silvermirrors are useful for application to solar reflectors, used outdoors.

With reference now to the drawing figures, wherein like numeralsrepresent like elements, there is generally shown the silver mirror 10,in FIG. 1, in its most basic configuration. The silver mirror 10 iscomprised of a polymeric substrate 11 having a thin, vapor-deposited,silver overlay 13, which exhibits a specular reflectance. Bonded to thesurface interface 14, of the silver layer 13, is a thin layer of afilm-forming protective polymer 15. Long-term durability and theretention of specular optical-efficiency and aesthetic appearance isprovided through the incorporation of a protective shield overlay 17,comprising a transparent multi-polymer film, such as acrylic, thatincorporates UV absorbers. This film 17 also shields the reflectiveinterface 14 and the underlying adhesives from UV degradation.Lamination of the UV absorbing polymer 17 to the protective film-formingpolymer 15 is done by means of an optical quality adhesive, a solventweld, or ultrasonic weld at interface 19. These adherent means arecharacterized by good adhesion, high optical performance, anddurability. Both the supersubstrate 17, and the film-forming polymer 15,are sufficiently transparent to visible light such that the silver 13specular surface may be observed and the high inherent reflectivity ofthe silver 13 fully utilized.

Referring now to FIG. 2, mirror 20, of the drawing, is a preferredembodiment of the invention. In this embodiment, silver layer 13 isvapor-deposited on the smooth-surface of a coextruded biaxially orientedpolyester foil 21, which consists of a polyethylene terephthalate lamina21B having conventional slip-agents, and a polyethylene terephthalatelamina 21A which does not contains a slip-agent resulting in an exposedsurface that is optically smooth. On the opposite surface of lamina 21Bis normally tacky and pressure-sensitive adhesive layer 27, which inturn is protectively covered by a release liner 18. For application ofthe mirror 20 to a solar reflector, the release liner 18 is removed andthe adhesive layer 27 is used to firmly adhere the mirror 20 to thesolar reflector base (not shown).

In the figure, the layered film-assembly 26 of mirror 20 may be, forexample, a high performance specular silver reflective film, such as theSILVERLUX polyester film, which is manufactured by the 3M Company underthe trade designation SS-95P. The SILVERLUX film 26 is conventional inconstruction and is commercially available with a removable premask (notshown) to protect the silver surface 13 prior to application. Layer 15is a thin acrylate coating that incorporates a corrosion inhibitor andlayer 16 is another thin acrylate overcoat that incorporates UVabsorbers. The SILVERLUX film 26 uses a pressure-sensitive adhesivelayer 27 for ease in permanent application to most smooth non-poroussubstrates, and an adhesive 27, which is protected by a release liner18. The two thin acrylate films 15 and 16, of assembly 26, tend,however, to weather poorly and to quickly erode away when used outdoorsas solar mirrors. Assembly 26 incorporates a polyester support sheet 21and a layer of silver 13. The presently preferred material for thesubstrate 21 is, as previously indicated, polyethylene terephthalate,which is susceptible to ultraviolet light degradation upon weathering.Ultraviolet light causes the polyester support sheet 21 to degrade andthe reflective film 26 too eventually lose specular reflectivity duringaccelerated weathering.

This degradation and loss of specular reflectance is, however,substantially overcome through bonding an overlay of a highlytransparent acrylic multipolymer film 17, containing UV absorbers, tothe surface of film 26. Disposed between the second acrylate layer 16and the acrylic multi-polymer film 17 is a bonding interface 29. In FIG.2, an adhesive 31 may is used to bond layers 16 and 17. The adhesive 31is highly optically transmissible to visible, ultraviolet, and nearinfra-red light. The acrylic multipolymer film 17, with UV absorbersincorporated therein, is a durable weather resistant acrylic shieldwhich retains an adequate strength, for its function, and originalaesthetics with outdoor exposure over time. The acrylate polymer film 17may be, for example, the KORAD KLEAR high gloss acrylic film, which iscommercially available under the trade designations KORAD KLEAR 05005 ina roll thickness of 2–8 mil, or the 3M Company extruded acrylic filmsold under the trade designation 3M X09105, which is 3.5 mil thick.

Turning now to FIG. 3, mirror 30 is another preferred embodiment of theinvention. Here, as in FIG. 2, disposed between second polymeric layer16 and acrylate polymeric layer 17 is a bonding interface 29. However,unlike FIG. 2, layer 16 and acrylic polymer film 17 are bonded togetherby means of a solvent or ultrasonic weld, which thereby eliminates theneed for the adhesive compound 31 of FIG. 2. While the weld at interface29 is preferably provided by an ethanol organic solvent, it isunderstood that other organic solvents, such as acetone, having likeproperties would also be suitable for bonding layers 16 and 17.

EXPERIMENT

In parallel runs, separate samples of KORAD KLEAR 05005 (1¾*2⅝ inches)were solvent-welded, with ethanol, over a SILVERLUX reflective film.This method resulted in a silver mirror according to the constructionset forth in FIG. 3, of the drawings. The ethanol solvent weld, in peeltests, resulted in an extremely adherent bond. Referring now to FIG. 4,it is shown a plot of the measured Spectral Hemispherical Reflectance,as a function of wavelength, for the KORAD KLEAR 05005 filmsolvent-welded to the SILVERLUX film. As shown in the figure, thesolvent-welded KORAD KLEAR 05005 and SILVERLUX reflector constructionresulted in a silver mirror, which retained its high optical claritythroughout the UV and visible spectrum.

The foregoing description is considered as illustrative only of theprinciples of the invention. Furthermore, since numerous modificationsand changes will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and processshown as described above. Accordingly, all suitable modifications andequivalents may be resorted to falling within the scope of the inventionas defined by the claims which follow.

1. In a silver mirror, comprising a polymeric substrate, aspecular-reflective silver layer overlying and bonded to said substrate,a protective layer of a transparent film-forming acrylate polymerincorporating a corrosion inhibitor and overlying and bonded to anexposed surface of said silver layer, and a second acrylate polymerincorporating a UV inhibitor bonded to said protective layer, theimprovement comprising a corrosion and ultraviolet-resistant- protectiveshield layer on said second acrylate polymer, said protective shieldlayer comprising: an overlay of a transparent multi-polymer film of athickness range of 2–8 mil that incorporates a UV absorber, and isadhered to an exposed surface of said second acrylate polymer or to anadhesive layer on said second acrylate polymer that is highly opticallytransmissible to visible, ultraviolet, and near infrared light to enablesaid silver layer to retain spectral hemispherical reflectance andoptical clarity throughout the UV and visible spectrum when used insolar reflectors.
 2. The silver mirror of claim 1, wherein theultraviolet incorporated transparent multipolymer film is an acrylicpolymer.
 3. The silver mirror of claim 1, wherein the ultravioletincorporated transparent multipolymer film is selected from the groupconsisting of polycarbonate, polyester, polyethylene naphthalate or afluoropolymer.
 4. The silver mirror of claim 1, wherein the ultravioletincorporated transparent multipolymer film is adhered to the exposedsurface of the second acrylate polymer by a solvent weld.
 5. The silvermirror of claim 1, wherein the ultraviolet incorporated transparentmultipolymer film is adhered to the exposed surface of the secondacrylate polymer by a thermal weld.
 6. The silver mirror of claim 1,wherein the ultraviolet incorporated transparent multipolymer film isadhered to the exposed surface of the protective layer by an ultrasonicweld.
 7. A method for making a silver mirror, comprising: (a) providinga polymeric substrate; (b) bonding a specular-reflective silver layer tosaid polymeric substrate; (c) bonding a protective layer of atransparent film-forming acrylate polymer incorporating a corrosioninhibitor to said silver layer and bonding a second acrylate polymerincorporating a UV inhibitor to said protective layer; and (d) adheringa protective shield layer to an exposed surface of said second acrylatepolymer or to an adhesive layer on said second acrylate polymer that ishighly optically transmissible to visible, ultraviolet, and nearinfrared light; said protective shield layer comprising a transparentmultipolymer film of a thickness range of 2–8 mil that incorporates a UVabsorber that enables said silver layer to retain spectral hemisphericalreflectance and high optical clarity throughout the UV and visiblespectrum when used in solar reflectors.
 8. The method of claim 7,wherein the ultraviolet incorporated transparent multipolymer isacrylic.
 9. The method of claim 7, wherein the ultraviolet incorporatedtransparent multipolymer is selected from the group consisting ofpolycarbonate, polyester, polyethylene naphthalate or fluoropolymer.